Are TB10s not as fuel efficient as PA 28s?
Is the premium for the GT models worth it?
Advice on best target year to go after in a TB10 (if models vary by year)
Do these airplanes command a premium over other similar used pistons (or the same or lower than others)
The TB10 is similarly fuel efficient to a PA28 of similar performance. It has a more spacious cockpit and this costs a bit of fuel compared to narrow cockpit types - there is no free lunch in aircraft design.
Re the GT premium - what you are really getting is two things: a newer plane which will give you more substantially maintenance-free years, and various little things which were fixed in the GT. The GTs were made only after year 2000 and piston production stopped ~ 2002 so not many were made. Looking at the serial number data e.g.
here (edit the URL for the starting S/N as desired) not many TB10s were sold during those years - presumably due to Socata's overly inflated pricing coupled with a start of a downturn in GA sales generally. I'd always buy a GT over a pre-GT, for both reasons above, but it does cost a bit more. Many of the GTs still look and smell like new. A TB20GT (if you can stretch your budget to that) is a really super plane with a lot more capability.
Re best years - unsuprisingly I would avoid really old ones but not for any specific reasons as the airframe never changed very much in its basics. Just buy the newest you can afford, and the usual advice is to buy a plane which already has all avionics you need already fitted (because installing avionics is a mug's game financially).
Re a premium - yes the GT does hold its price better but there is very little data on the TB10GT because there are not many of them about. A TB20GT, year 2002, good condition etc, will sell for GBP 140-150k which is probably 2x more than a 1995 one of similar spec and post-overhaul engine hours would sell for. But realistically no plane holds its value well these days. Socatas do a lot better than Cirruses whose values become dust after a few years, but I now see early TB20s for GBP 40k... (OK those are real dogs, but a PA28 of same age would be a dog too).